A small experiment that could eventually make a big difference for the climate is taking place on farmer Myron Ortner's land in central Michigan.

On a 40-acre plot where he grows corn and soybeans in rotation, Ortner has worked with Michigan State University researchers for the past several years to reduce his nitrogen fertilizer use. Normally, Ortner said, farmers use about 200 pounds of nitrogen fertilizer per acre, but now he's down to about 135 pounds per acre on the test plot.

"I found out we can use less nitrogen and get away with it through those studies," Ortner said. "I want a few more years on it before I'm going to commit all my acres to it, but I don't think I've lost any yield by doing what we're doing."

For farmers trying to feed an increasingly hungry planet, nitrogen fertilizer is a good thing, significantly boosting their crop yields.

But when it comes to the climate, too much fertilizer can spell trouble, new research from Michigan State scientists recently confirmed. Any nitrogen not taken up by plants is met by soil microbes that turn the fertilizer into nitrous oxide, an ozone-depleting gas that is also 300 times more potent than carbon dioxide.

For this reason, it's important to figure out where—and by how much—farmers can afford to cut back on their nitrogen fertilizer use. The Michigan State study, released this week, sheds new light on this issue, showing that farmers applying more nitrogen than plants can use are likely having an outsized impact on the climate.

The research, published in the journal Proceedings of the National Academy of Sciences, brings together dozens of studies from across the globe that analyzed how nitrous oxide emissions increased based on different rates of fertilizer application.

"What we found is that it's not a linear relationship between the nitrous oxide and the [nitrogen fertilizer] rate applied, but a nonlinear, exponential trend across many different crop types," including corn, major grain types, rice and grasses, said study co-author Neville Millar, senior research associate with Michigan State's W.K. Kellogg Biological Station.

"Nitrogen is essential for many crops in current agricultural systems, but frequently too much is applied for crop needs, leading to larger emission of nitrous oxide into the atmosphere," Millar added.

Soil nitrogen expert Eric Davidson, a scientist at the Woods Hole Research Center in Falmouth, Mass., who was not involved in the study, said the finding is important because it focuses scientists, policymakers and farmers on making reductions where they will make the most difference.

"African farmers who have not been able to afford fertilizers could start using them without much danger of increasing N2O emissions," Davidson said in an email. "Chinese farmers, who sometimes use 2-4 times as much fertilizer as American farmers, could reduce their nitrous oxide emissions substantially by cutting back even just a little off of their large application rates."

"At the local or regional scale, distinguishing between linear and nonlinear responses is very important for figuring out where we can get the most bang for our greenhouse gas mitigation buck and where we can dare to use more fertilizers to improve food security and reduce malnutrition," Davison added.

Leap of faith for farmers
Millar stressed that the research is not meant to persuade all farms to cut back on nitrogen fertilizer—just those that are probably using too much.

"We're very cognizant of the fact that yields are very important—essential to farmers, essential to the growing population," Millar said.

According to study co-author Philip Robertson, who directs the Kellogg Biological Station, there are a number of proven ways to reduce farmers' nitrogen use. For example, they can apply fertilizer in the spring instead of the fall so that much of the nitrogen is not lost before the seeds begin growing. They can also place the fertilizer deeper in the soil, where plants can access it more easily.

But getting farms to make these changes can be a challenge, Robertson said. Farmers often apply the amount of nitrogen recommended by fertilizer companies,

"If you don't get it right, your yields will suffer," Robertson said. "So it takes some faith in the science that is underlying fertilizer recommendations."

One new method urging farmers to take this leap of faith is a voluntary carbon market, where credits are issued by a carbon offset standard organization called the American Carbon Registry. With the help of the Delta Institute and the Climate Trust, nonprofits focused on market-driven emissions reduction efforts, offsets are paid out to farmers when they document reductions in their nitrogen fertilizer use.

In return for his efforts on his 40-acre Michigan plot, this week the Carbon Registry issued Ortner the first offsets as part of this program, based on a verification method developed by Millar and colleagues with the financial support of the Electric Power Research Institute.

"Simply by adding less fertilizer, he did not reduce his yields, he saved some on fertilizer costs and he garnered the credits that can be sold for additional income," Robertson said.

Prices on the voluntary carbon market are likely too low for this method to attract the interest of most U.S. farmers in the near term, Robertson said. But if policies are enacted that cause greenhouse gas credits to rise in value, Robertson said, "I'm hoping ... that this will be sufficient to nudge farmers into taking the extra trouble to apply [nitrogen] more judiciously."

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